Mass spectrometry



March 5, 1957 C, F, ROBlNsQN 2,784,317

MASS SPECTROMETRY Filed Oct. 28, 1954 5A W TOOTH OSC/LLATOR POWER SUPPLY INVENTOR. CHARLES E' ROBINSON A TTORNEVS `United States Patent 2,1's4,1i MASS' sPEcrRoMETRY Charles F. Robinson, Pasadena, Calif., assignor, by mesne assignments, to Consolidated Electrodynamics Corporation, Pasadena, Calif., a corporation of California Application october 2s, 1954, serial No. 465,247 s Claims. (cl. 25o-41.9)

This invention relates to mass spectrometnl and more particularly to an improved instrument' of the typeY which has become known as a linear R. F. or time of flight mass spectrometer.-

" Uln mass spectrometry a sample of material to be ana*- 'lyzed is ionized by any one of several known means and theSions-thus formed are separated as a? function of massl tocharge ratio. Separation of theions in accordance with mass to charge ratio is generally accomplished under the inuence of electric or magnetic elds, or both, to effect spatial separation thereof. Ions of a given mass to charge ratio may then be directed upon anion collector or target and discharged whereby a measure of the resulting current maybe obtained.

In a linear R. F. mass spectrometer separation of ions is accomplished by taking advantage of the characteristic velocities imposed on ionsv of differing mass to charge ratiov by an accelerating electrical iield. Generally the ions, after formation or upon formation, are subjectedto the inuence of a propelling electrical field and are caused to travel along-a so-called drift tubeV wherein the ion masses separate as a consequence of their characteristic and-differing response to their imposed acceleration. 1 One particular type of linear R. F. mass spectrometer employs: bunching grids'adjacent thepoint of ionform'ationwhereby ions are bunched as they proceed alongsthe drift tube. ln this manner ions of diiere'nt mass arrive atfthe collector electrode at different times,vso controlled that the time duration of they individual bunches'isfin the firstorder approximately. zero. In this type of ins/trumenta gate grid is conventionally employedadjacent the collector electrode and so triggered with respect tothe acceleration system that ions of a preselectedmass and hence a preselected transit time are passedithrough tothe collector, to the exclusion of other ions;

g However, it has been found that in suchA an instrument an ion approaching a collector induces a current in the collector given'by:

where I is the instantaneous value of the induced collector curq is the charge on the'iomand Cis the capacity coeicientY between the ion and thecollector.:

2,784,317 referred Mar. `5, 1957 only if it is arranged so that at all times dCi/dt=0 for all bunches except one. The satisfaction of this requirement has been shown to be very ditlicult in a small instrument. Alternatively, the ion collector may be precluded from seeing more than one bunch of ions by means of a gating grid systeml shielding the collector. Y Such a system is described in United States patent application Serial No. 408,879, tiled February 8, 1954, by Robert V. Langmuir.

I have now developed a time ofight or linear R. F. mass spectrometer in which the need for so-called gating or shield grids is eliminated with the resultant advantages of a more compact and simpler instrument, and the further advantage of ability to display the' entire spectrum on a cathode ray tube. This improved collection is accomplished in accordance with my invention` by replacing the gating system, ion collectorA and amplification system of a conventional time of flight mass spectrometer with an electron multipler. The invention accordingly' contemplates in a mass spectrometer, the combination comprising an evacuable envelope, means for developing ions within a restricted region of the envelope, an electron multiplier disposed in the envelope remote from the regionV of ion formation, and means for periodically propelling ions4 from the restricted region toward the' electron multiplier so that ions fromk each propulsion period strike the rst dynode of the electron multiplier in timed, spaced bunches of successively diferent mass. l

ln such a mass spectrometer the limitations established by Equations l and 2 above disappear, since it is not the current induced on the lirst dynode of the electron multi'- plier by the moving ions which is measured but the' electrons ejected from itby actual impact of the ions. In other words, capacity currents tothe first dynode have no effect on the output of the electron multiplier and as a consequence induced current which complicates' the use ofa collector electrode in the conventional manner is no longer a problem. The principal advantage' in" present instrument is that no objection is presented by the fact that the electron multiplier can see more than one bunch of ions at a time, whereas with a conventional instrument if the ion collector electrode can see more' than one bunchy ofv ions at a time the instrument is in' eiect inoperable. By reason of the fact that no harm results from the ability of the electron multiplier of the present instrument to see more than one bunch of ions i't is possible to eliminate the various intermediate gating grid systems previously proposed with a consequent result of a more compact simplerv instrument.

, The electron multiplier arrangement herein proposed has a further advantage in that the entire spectrum under analysis can be simultaneously displayed on a cathode ray tube connected in the output of the last dynode o f the multiplier. Such display is impossible in the converttional instrument for theV reason that induced currents in the collection system attendant upon any attempt to' so operate the collector electrodel as to see the entire' ion spectrum, defeats the operation of the instrument. IfA the spectrum is gated as isv required with a conventional: collector electrode then only one ionmass at a time can-ille displayed on an associated cathode ray tube.V

Av mass spectrometer in accordance with the invention is described in greater detail in relation to the accompanying. drawing. which is a schematic longitudinal section of a linear R. F. mass spectrometer.

The mass spectrometer in the drawing comprisesl an evacuable tube 1 provided with: an outlet 2l for con'- nection to an evacuating system (not'shown). andan inlet 3 for introduction of a gaseous sampler from a source (not shown). An ion source=|4 is; disposed: at one end of the tube I and is connected to the inlet-5 for introduction of 'tle sample directly into the ion source.

kion formation. a solid sample, electron gun 8 may be replaced by an Electrodes 5, 6, v7 are mounted in the source, electrodes v 6 and 7 being in the form of grids or perforate plates to permit passage of ions therethrough. A conventional yelectron gun 8 illustrated as anelectron emitting cathode Vis mounted adjacent the wall of the source 4 and is positioned to direct electrons through a lirst aperture 9 in the source wall to traverse the source as a beam 10 and pass through a diametrically opposed aperture 11 in the `source wall to strike and discharge at an electron target 12. A control gird 14 is interposed between the electron gun 8 and the source 4 and by means of which electron flow from the gun to the target 12 can be controlled as hereinafter explained.

' An electron multiplier 16 is mounted at an end of tube 10 opposite the source 4, the several dynodes 17,

18, 19 of the multiplier being connected for energization to a Voltage divider 20 which is in turn connected across va voltage source 21. A cathoeray tube 22 is connected to the terminal dynode 19 of the multiplier 16 across a shunt resistor 23 whereby the signal developed at the dynode 19 may be displayed on the tube 22.

The operation of the ion source is controlled by a circuit including a variable frequency saw-tooth oscillator 24 and a high voltage power supply 25 which is connected to be controlled by the oscillator 24. The output of the high voltage power supply 25 is connected through a bias battery 26 across a voltage divider 27. At appropriate intervals along the divider 27 electrodes 5, 6 and 7 are tapped so that within the ion source 4 a periodic vaccelerating eld of saw-tooth voltage configuration is applied to the ions formed in the electron beam 10. Electron gun 8 is connected through battery 28 to the divider 27 and control grid 14 is connected through a potentiometer network 29 to the gun 8. Any conventional circuitry can be employed in connection with the electron gun, control grid and target.

Also included in the instrument shown in the drawing are a series of three so-called focusing electrodes 30, 31,

32, electrodes 30 and 31 being grounded to the tube 1 Aand electrode 32 being provided with a small positive potential. Although the focusing electrodes are not essential to practice of the invention they represent a preferred expedient and operate to improve the convergent focus of ions traveling in the tube 1 on the iirst dynode 17 ofthe electron multiplier.

The operation of the instrument as illustrated in the drawing is as follows: A gas sample admitted into the ion source through the sample inlet tube is ionized at least in part by the electron beam directed across the source. Ion formation by means of an electron beam is quite common, although it is to be understood the instrument of the present invention is not in any way limited in its application to this particular manner of For example, if it is desired to analyze emitter of the sample whereby emitted ions will be directed under the inuence of control grid 14 into the region wherein acceleration takes place in the source as illustrated. Under such conditions the potential on con- .trol grid 14 would be altered to control the llow of emitted ions.

A saw-tooth voltage pulse derived from the voltage :supply network shown schematically as the saw-tooth oscillator and high voltage power supply is applied, in

nproportion to the setting of voltage divider 27, to the is described in co-pending United States patent application Serial No. 408,654, tiled February 8, 1954, by Lawrence G. Hall and Harold W. Washburn. Operation of an ion source in accordance with the teaching of this co-pending application is highly satisfactory in the instrument of the present invention although not an essential element thereof. As the ions discharged from the source travel along the dritt tube 1 they segregate themselves into spatially separated bunches with each bunch constituting ions of a given specific mass differing from the mass of ions of the other bunches; this effect resulting from the fact that an ions response to an accelerating influence is a function of the mass of the ion. Under the influence of the focusing electrode array 30, 31 and 32 the ion bunches are caused to converge at a focal point corresponding to the initial dynode 17 of the electron multiplier 16.

As previously mentioned by reason of the fact that an electron multiplier operates on a principle of secondary emission, its output is completely insensitive to the approach of succeeding ion bunches until such bunches actually strike the dynode 17. These succeeding bunches can thus be accepted at the electron multiplier in rapid succession without the need of any gating or triggering system and the entire ion spectrum developed in the cathode ray tube 22 after amplification through the dynode array of the electron multiplier.

The invention is specifically directed to the use of an electron multiplier type system in a linear R. F. or time of flight mass spectrometer for the purpose of avoiding the inherent limitations of a conventional collector electrode with respect to its sensitivity to induced currents and correlatively eliminating the need for gating systems which greatly increase the complexity of the instrument due to the problems of synchronization between the accelerating and gating functions.

I claim:

1. In a mass spectrometer the combination comprising an evacuable envelope, means for developing ions within a restricted region of the envelope, an electron multiplier disposed in the envelope remote from the region of ion formation, and means for periodically propelling ions Y from the restricted region toward the electron multiplier by meansof accelerating voltage pulses, the amplitude of each of which varies during the duration of the pulse so that the time duration of each successive ion train at the multiplier is substantially shorter than the time duration of the accelerating voltage pulse producing it.

2. In a mass spectrometer the combination comprising an evacuable envelope, means for developing ions within a restricted region of the envelope, an electron multiplier disposed in the envelope remote from the region of ion formation, means for periodically propelling ions from the restricted region toward the electron multiplier by means of accelerating voltage pulses, the arnplitude of each of which varies during the duration of the pulse so that the time duration of each successive ion train at the multiplier is substantially shorter than the time duration of the laccelerating voltage pulse producing it, and a cathode ray tube connected to display,l the output signal of the electron multiplier. A 3. In a mass spectrometer the-combination comprising an evacuable envelope, means for developing ions within a restricted region of the envelope, an electron multiplier disposedin the envelope remote from the region of ion formation, means for periodically propelling ions from the restricted region toward the electron multiplier by means of accelerating voltage pulses,` the amV plitude of each of which varies during the duration of the pulse so that the time duration of each successive ion train at the multiplier is substantially shorter than the time duration of the accelerating voltage pulse producing it, and focusing means disposed in the envelope between the source of ions and the electron multiplier for focusing the ions on the rst dynode of the multiplier.

4. In a mass spectrometer the combination comprising an evacuable envelope, means for developing ions within a restricted region of the envelope, an electron multiplier disposed in the envelope remote from the region of ion formation, means for periodically propelling ions from the restricted region toward the electron multiplier by means of accelerating voltage pulses, the amplitude of each of which varies during the duration of the pulse so that the time duration of each successive ion train at the multiplier is substantially shorter than the time duration of the accelerating voltage pulse pro ducing it, a cathode ray tube connected to display the output signal of the multiplier, and focusing means disposed in the envelope for focusing the ions on the rst dynode of the multiplier.

5. Apparatus according to claim 3 wherein the focusing means comprises a series of three electrodes and means biasing the center one of the three electrodes positively with respect to the outer two.

6 References Cited in the le of this patent UNITED STATES PATENTS 2,582,216 Koppius Jan. 15, 1952 2,664,515 Smith Dec. 29, 1953 2,696,561 Schroeder Dec. 7, 1954 OTHER REFERENCES A Pulsed Mass Spectrometer With Time Dispersion, by Wolf -and Stephens, published in Review of Scientific Instruments, vol. 24, No. 8 (August 1953), pages 616, 617. Copy in Patent Oce Library.

An Ion Velocitron, by Cameron and Eggers, published in The Review of Scientific Instruments, vol. 19, No. 9 (September 1948), pages 605-607. Copy in Patent Office Library.

The Detection of Single Positive Ions, Electrons and Photons by a Secondary Electron Multiplier, by James Allen, published in Physical Review, vol. (May l5, 1939), pages 966-971. Copy in Patent Oflice Library. 

